Lenticular energy metabolism during exogenous calcium deprivation and during recovery: effects of dextran-40

ATP, the 31P Spectral Modulus, and Metabolism

Adenosine triphosphate (ATP) has a high intracellular millimolar concentration (ca. 2.4 mM) throughout the phylogenetic spectrum of eukaryotes, archaea, and prokaryotes. In addition, the function of ATP as a hydrotrope in the prevention of protein aggregation and maintenance of protein solubilization is essential to cellular, tissue, and organ homeostasis. The 31P spectral modulus (PSM) is a measure of the health status of cell, tissue, and organ systems, as well as of ATP, and it is based on in vivo 31P nuclear magnetic resonance (31P NMR) spectra. The PSM is calculated by dividing the area of the 31P NMR integral curve representing the high-energy phosphates by that of the low-energy phosphates. Unlike the difficulties encountered in measuring organophosphates such as ATP or any other phosphorylated metabolites in a conventional 31P NMR spectrum or in processed tissue samples, in vivo PSM measurements are possible with NMR surface-coil technology. The PSM does not rely on the resolution of individual metabolite signals but uses the total area derived from each of the NMR integral curves of the above-described spectral regions. Calculation is based on a simple ratio of the high- and low-energy phosphate bands, which are conveniently arranged in the high- and low-field portions of the 31P NMR spectrum. In practice, there is essentially no signal overlap between these two regions, with the dividing point being ca. -3 δ. ATP is the principal contributor to the maintenance of an elevated PSM that is typically observed in healthy systems. The purpose of this study is to demonstrate that (1) in general, the higher the metabolic activity, the higher the 31P spectral modulus, and (2) the modulus calculation does not require highly resolved 31P spectral signals and thus can even be used with reduced signal-to-noise spectra such as those detected as a result of in vivo analyses or those that may be obtained during a clinical MRI examination. With increasing metabolic stress or maturation of metabolic disease in cells, tissues, or organ systems, the PSM index declines; alternatively, with decreasing stress or resolution of disease states, the PSM increases. The PSM can serve to monitor normal homeostasis as a diagnostic tool and may be used to monitor disease processes with and without interventional treatment.

Hydrotropic function of ATP in the crystalline lens

The hypothesis proposed herein is presented to explain the unexpectedly high concentration of ATP and provide evidence to support its hydrotropic function in the crystalline lens determined using ³¹P NMR. The lens, historically considered to be a metabolically quiescent organ, has the requisite machinery to synthesize ATP, such that the homeostatic level is maintained at about 3 mM. This relatively high concentration of ATP has been found to be consistent among multiple mammalian species including humans. This millimolar quantity is many times greater than the micromolar amounts required for the other known functions of ATP. The recent postulation that ATP at millimolar concentrations functions as a hydrotrope in various cell/tissue homogenates preventing protein aggregation coupled with observations presented herein, provide support for extending the hypothesis that ATP functions as a hydrotrope not only in homogenates but in an intact functioning organ, the crystalline lens. Concentrations of ATP of this magnitude are hypothesized to be required to maintain protein solubility and effectively prevent protein aggregation. This concept is important considering protein aggregation is the etiology for age-related cataractogenesis. ATP is a common ubiquitous intracellular molecule possessing the requisite hydrotropic properties for maintaining intracellular proteins in a fluid, non-aggregated state. It is proposed that the amphiphilic ATP molecule shields the hydrophobic regions on intralenticular fiber cell protein molecules and provides a hydrophilic interfacial surface comprised of the ATP negatively charged triphosphate side chain. Evidence is presented that this side chain is exposed to and has been reported to organize intracellular interstitial water to form an interfacial rheologically dynamic water layer. Such organization of water is substantiated with the effect of deuterium oxide (heavy water) on ATP line widths of the side chain phosphates measured ex vivo by ³¹P NMR. A novel model is presented to propose how this water layer separates adjacent lens fiber cell proteins, keeping them from aggregating. This hypothesis proposes that ATP can prevent protein aggregation in normal intact lenses, and with declining concentrations can be related to the disease process in age-related cataractogenesis, an affliction that affects every older human being.

The effects of age on phosphatic metabolites of the human crystalline lens

Paired human lenses ranging from 2 to 96 yr old (n = 17) were harvested within 2 hr post-mortem and immersed in liquid nitrogen. Phosphorus containing metabolites from perchloric acid extracts were quantitated using phosphorus-31 nuclear magnetic resonance spectroscopy. Low-energy metabolites detected include the phosphomonoesters (PME) hexose-6 phosphate, alpha-glycerol phosphate, fructose 1,6-diphosphate, beta-glycerol phosphate (beta-GP), uncharacterized phosphomonoesters at 4.63 delta, 4.34 delta and 3.05 delta, phosphorylethanolamine (PE), inosine and adenosine monophosphates, and phosphorylcholine (PC). Inorganic orthophosphate (Pi), glucose 1-phosphate and glycerol 3-phosphorylcholine were the other low-energy metabolites detected. High-energy metabolites phosphocreatine (PCr), adenosine tri- and di-phosphates (ATP, ADP), nucleoside diphosphorylsugars (NS) and the dinucleotides were also detected. The following metabolic indices were calculated: PCr/Pi, PME/Pi; ATP/Pi, PCr/ATP, ATP/ADP, PE + PC, PC/PE, energy charge, phosphorylation potential, and the energy modulus. A significant linear increase with age (P less than 0.05) occurred only in the uncharacterized resonance at 3.05 delta (r = 0.4593). Significant linear decreases with age (P less than 0.05) occurred in the uncharacterized resonance at 4.63 delta (r = -0.5950), beta-GP (r = -0.5018), PCr (r = -0.4495), N.S. (r = -0.4882). There was a significant (P less than 0.05) linear decrease in the energy modulus (ratio of high-energy to low-energy metabolites).

Phosphorus-31 NMR study of the effects of hydrogen peroxide on young and old rat lenses

31P-NMR spectroscopy was used to study the dynamic changes in young and old rat lenses under oxidative stress with hydrogen peroxide. Control spectra were recorded for young and old rat lenses using normal media. Oxidative stress spectra were recorded under the same conditions, except that the normal media also contained hydrogen peroxide at four different concentrations. With increasing H2O2 concentration in the perfusion media there was a corresponding decrease in the observed phosphorus metabolites, phosphorylcholine and ATP. There was significant difference in the rate of depletion of metabolites between the young and old rat lenses; old rat lens showed an ATP decrease almost double that for young rat lenses. Also, the 31P control spectrum of the old rat lens was different from that of the young lens. The NMR results showed the importance of comparison of old and young rat lenses under oxidative stress as a model for senile cataractogenesis.

Effects of the slow calcium-channel blocker verapamil on phosphatic metabolism of crystalline lens

The effects of calcium-channel blockade on phosphate metabolism in the rabbit lens ex vivo was studied using phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. Rates-of-change of the intralenticular pH and the following phosphatic metabolites in the lens were determined: ATP, ADP, inorganic orthophosphate, sugar phosphates, nucleoside diphosphosugars, phosphodiesters, dinucleotides, and an uncharacterized resonance peak at 6 delta in the 31P spectrum. Incubation in 20, 200 or 2000 micrograms/ml verapamil led to the same qualitative changes in the lens's metabolic profile at each different concentration; only the rates of metabolite change were altered significantly by verapamil concentration. Compared with the control, the entire orthophosphate resonance band increased relative to the signals of ATP, and the alpha-glycerophosphate and nucleoside monophosphate resonances increased in relation to inorganic orthophosphate, and these two resonance groups shifted relative to each other so that they coalesced, indicating that the two groups of signals arose from compounds in relatively different pH compartments. In addition to altering metabolic rates, verapamil reduced the intralenticular pH of the sugar phosphate and inorganic orthophosphate pools with respect to those containing the nucleoside monophosphates; the pH-change difference was about one order of magnitude smaller for the mononucleotide pool. Despite the metabolic alterations and changes in pH during verapamil incubation, lens clarity was unchanged.

Dynamic phosphorus-31 NMR study of sugar cataract and its prevention in rat lenses with aldose reductase inhibitors

P31 NMR spectroscopy was used to study dynamic changes in rat lens phosphate metabolites under hyperglycemic conditions. NMR control spectra were collected at 37 degrees C with normal media; NMR spectra were also collected under the same conditions except the normal media was substituted for one containing 30 mM xylose. There was a significant increase in intensity of the sugar phosphate NMR signal as well as a moderate decrease in intensity for the three ATP peaks. In separate experiments normal rat lenses were pre-treated with perfusion media containing the ARIs; either Spiro-(2-fluorofluorene-9,4'-imidazolidine)-2',5'-dione (AL theta 1567) or Spiro-(2,7-difluorofluorene-9,4'-imidazolidine)-2',5'-dione (AL theta 1576) for three hours before changing to a xylose plus ARI media. After overnight perfusion, no appreciable change in the rat lens NMR spectra was found. This NMR observation indicates that lenses can be protected from hyperglycemic stress in the presence of the ARIs AL theta 1567 or AL theta 1576.

Phosphorus nuclear magnetic resonance and ocular metabolism

Phosphorus (31P) nuclear magnetic resonance (NMR) represents a noninvasive technique for the assessment of ocular metabolism. The measurement of a spectrum of phosphorus-containing metabolites (e.g., phosphorylated sugars and ATP), including a number of heretofore uncharacterized metabolites, can be made with a single analysis. In addition to quantitating phosphatic metabolites, 31P NMR can be employed to monitor (1) the rate of metabolic change in a specific biochemical reaction via T1 and T2 relaxation times, and (2) the rate of change in the concentration of a particular metabolite. Several calculations indicating tissue energy status (health) can be made using quantitative spectroscopic information including: the phosphorylation potential, the energy charge of the adenylate system, and the 31P spectral modulus. Tissue pH can be determined as a function of shift in 31P NMR signals. 31P NMR techniques have both research and diagnostic applications in ophthalmology since potentially it provides a noninvasive method to analyze ocular tissues metabolically and detect subtle biochemical changes that precede overt manifestation of disease states. Such detection may allow for early and more effective therapeutic intervention of disease. Furthermore, the noninvasive quality of NMR spectroscopy will permit continual evaluation of therapy.